In middle and late adulthood, all people experience a series of progressive alterations in body composition. The lean body mass shrinks and the mass of adipose tissue expands. The contraction in lean body mass reflects atrophic processes in skeletal muscle, liver, kidney, spleen, skin and bone.
These structural changes have been considered unavoidable results of the aging process. It has recently been proposed, however, that reduced availability of growth hormone in late adulthood may contribute to such changes. For example, after about the age of 30, the secretion of growth hormone by the pituitary gland tends to decline.
Pituitary growth hormone is a classical endocrine hormone which has profound effects on somatic growth and body composition. Growth hormone secretion is both pulsatile and diurnal. It is, thus, difficult to measure the 24-hour secretion of the hormone directly.
Growth hormone secretion, however, can be measured indirectly by measuring the serum concentration of insulin-like growth factor 1 (IGF-1; also known as somatomedin C) which is produced and released by the liver and perhaps other tissues in response to growth hormone, and which serves as an indicator of overall growth hormone secretion. Serum IGF-1 concentrations increase in response to both endogenously and exogenously administered growth hormone, have little diurnal variation, and are low in the case of growth hormone deficiency. Serum IGF-1 concentrations also decline with advancing age in healthy adults. Less than five percent of healthy men 20 to 40 years old have serum IGF-1 values below 350 U per liter (1 unit=240 ng); however, 30 percent of healthy men over 60 have values below this figure. Decline, with advanced age, of serum IFG-1 concentration has been correlated with the decline or disappearance of the nocturnal pulses of growth hormone secretion. If the serum concentration of IGF-1 falls below about 350 U per liter in older adults, no spontaneous circulating pulses of growth hormone can be detected by currently available radioimmunoassay methods. The concomitant decline in serum concentrations of both hormones supports the view that the decrease in IGF-1 results from diminished growth hormone secretion. At all ages, it has been found that serum level of IGF-1 is inversely correlated with adiposity.
Diminished secretion of growth hormone is accompanied not only by a fall in the serum IGF-1 concentration, but also by atrophy of the lean body mass and expansion of the mass of adipose tissue. These alterations in body composition caused by growth hormone deficiency can be reversed by replacement doses of the hormone, as shown by experiments in rodents, children and adults 20 to 50 years old. These findings suggest that the atrophy of the lean body mass and its component organs and the enlargement of the mass of adipose tissue that are characteristic of the elderly result, at least, in part, from diminished secretion of growth hormone. If so, age-related changes in the body composition should be correctable, in part, by the administration of human growth hormone, now readily available as a biosynthetic product. Several studies have now confirmed that beneficial results can be obtained from growth hormone treatment. See, for example, Rudman et al., New England Journal of Medicine, vol. 323 (1990) pp. 1-5, in which healthy aged men who had IGF-1 levels below 350 U/L were administered growth hormone. See, also, Salomon et al., N. Engl. J. Med., vol. 321 (1989) pp. 1797-1803; Cuneo et al., J. Appl. Physiol., vol. 70 (1991) pp. 688-694; Cuneo et al., J. Appl. Physiol., vol. 70 (1991) pp. 695-700 et al J. Clin. Endocrinol. Metab., vol. 70 (1990) pp. 519-527.
Much of the focus of such studies has centered on changes in muscle and bone associated with growth hormone levels. See, for example, Brixen et al., J. Bone Min. Res., vol. 5 (1990) pp. 609-618; Stracke et al., Acta Endocrinol., vol. 107 (1984) pp. 16-24; Chenu et al., Bone, vol. 11 (1990) pp. 81-86. At least one study suggests that IGF-1 itself has an independent effect on bone matrix formation; see, Hock et al., Endocrinol., vol. 122 (1988) pp. 254-260. It has been found that some bone deterioration associated with age, e.g., kyphosis, is irreversible, thus supporting a view that growth hormone might be beneficially used as a prophylactic.
One marker of bone remodeling/formation is the serum level of osteocalcin, a vitamin-K dependent protein synthesized in bone. Pun et al., Bone, , vol. 11 (1990) pp. 397-400. Measurement of circulating levels of osteocalcin can provide information on bone turnover. The Pun et al. study found a significant correlation between serum osteocalcin levels and IGF-1, indicating that IGF-1 is a determinant of serum osteocalcin.
While studies suggest potential benefits to body composition of growth hormone administration, some serious concerns remain regarding the effects of long term administration of growth hormone. Numerous studies suggest both beneficial and harmful effects on metabolic function. Toxic, or potentially toxic, side-effects of prolonged growth hormone treatment (i.e., daily administration over several weeks' time) which have been observed with the administration of moderate or high doses (i.e., doses above normal blood levels) include stimulation of neoplastic growth, acromegaly, amenorrhea, flushing and nausea, "dawn" phenomena (a condition of insulin insufficiency which is very dangerous for diabetics), either hyperglycemia or hypoglycemia in diabetics, and fluid retention caused by kidney glomeruli retention of sodium. (Scientific American Medical Information Text, Eds. E. Rubinstein and D. Fedman, Scientific American Soc. (1992)) Thus, similar to treatment with other hormones, long-term use is often a balancing act.
Moreover, efforts to administer IGF-1 rather than growth hormone have also raised concerns of harmful effects. Potential side-effects of prolonged (as defined above for growth hormone) treatment with moderate or high doses (as defined above for growth hormone) of IGF-1 include abdominal bloating, indigestion and nausea, and alteration in cellular insulin receptor sites. The latter effect could result in changes in glucose or fatty acid oxidation, as well as either insulin insufficiency or excess. Such effects are quite dangerous for diabetics. (Scientific American Medical Information Text, Eds. E. Rubinstein and D. Fedman, Scientific American Society (1992))
In addition to diminished growth hormone levels with increasing age, other studies have reported age-linked changes in the activity of enzymes and cofactors involved, for example, in the tricarboxylate cycle and lipid oxidation. One such cofactor is carnitine (.beta.-hydroxy-.gamma.-N-trimethylammounium butyrate). Carnitine is required for fatty acid oxidation a major source of energy for normal body function. Carnitine has two critical functions in the cell, namely, (1) to stimulate fatty acid oxidation by transporting acyl groups across the inner mitochondrial membrane, resulting in ATP formation, and (2) to remove extra or "toxic" acyl groups from the mitochondria and cell as carnitine esters.
Carnitine is present in both serum and urine in free and esterified forms, although in human beings carnitine esters are preferentially excreted while free carnitine is reabsorbed by the kidney. Normal total serum carnitine concentrations range from 31-79 .mu.M for men and 25-69 .mu.M for women, and free carnitine from 28-68 .mu.M for men and 21-57 .mu.M for women. Carnitine deficiency is defined as a total serum carnitine of 20 .mu.M or lower, or a free carnitine of 20 .mu.M or lower. The ratio of esterified carnitine, i.e., carnitine esters, to free carnitine, E/F, is a relatively good indicator of mitochondrial oxidation and ATP formation. An E/F ratio of 0.4 or higher is indicative of free carnitine insufficiency and indicates poor oxidation of fatty acids and low ATP production. An abnormally high E/F ratio may also indicate increased removal of toxic acyl groups because of a genetic enzyme defect, ingestion of potentially toxic compounds (e.g., valproic acid), or a generalized decrease in carnitine-related metabolism associated with, e.g., aging. Related animal studies have indicated that both lipid oxidation and carnitine levels are decreased in aged rats. (Hansford et al., Mechanisms of Aging and Dev., vol. 19 (1982) pp. 191-201.)
Carnitine has also been shown to be important in normal growth. (S. C. Winter et al., Am. J. Dis. Child. vol. 141 (1987) p. 660) For example, children with carnitine deficiency have stunted growth which normalizes with carnitine repletion.
Thus, the prior art teaches age-linked decreases in body levels of growth hormone. The prior art also teaches the use of growth hormone to retard bone loss associated with aging, and that IGF-1 concentration serves as an indicator for overall growth hormone secretion and that osteocalcin concentration is an indicator of bone formation. The art has, however, not provided any link between in vivo carnitine level and IGF-1 or bone loss.